After doing some testing, it seem to be quite random. Either random between all the plant (Survival endless) or random between a row (in the adventure).

What worked best was to play the adventure however and ever. In the level that correspond to row of the plant I am going, there is reasonable probability that I will have it after getting a few. Not sure, but improve the odd over playing survival endless.

as said before water plants are in pool levels and shrooms are in night levels.you get normal plants on your lawn.in i zombie levels(in the puzzle option) when you eat the brains it might give you a plant but it gives all kinds of plants also replay whack a zombie and bust up as many graves you can and at the end other than collecting the money bag destroy all graves,its also a good way to money too :)

I'll assume you are talking about plants that you find for your zen garden (the only other option is the plants unlocked while playing the game's adventure mode, and those have a set order which never changes).

There is no way to increase the probability of receiving a certain kind of plant for your zen garden. The type of plant received is completely random.

Looking around the internet, it is said that while there is an equal chance to get all types of plants on Survival: Endless, other minigames have a higher tendency to produce plants you first unlock in the area where the minigame is played (night levels will produce more night-time plants, for example). This is not backed up by any hard evidence, however, and I can't really say I've noticed this during the countless hours my wife strived to acquire every possible plant for her garden.

Since the game version isn't specified, it would be good to note that on the iOS versions (Maybe Android too, but I haven't had a chance to check), you can buy more expensive presents that have an increased chance of giving you a plant you don't already have, maxing at 100% chance of a new plant. While it's still random, this could help improve things, at least.
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ZAD-ManSep 4 '13 at 21:29